1. Field of the Invention
The present invention relates to a steering shaft which is telescopically contractible to secure the safety of a driver during an accident, such as a collision of a vehicle.
2. Description of the Related Art
Conventionally, a steering shaft which is made telescopically contractible to secure the safety of a driver during an accident, such as a collision of a vehicle, is known as the steering shaft of a steering apparatus. This steering shaft is arranged such that corresponding end portions of a plurality of shafts are fitted over and coupled to each other by means of splines such that the shafts are slidable relative to each other in their axial direction.
Further, a steering shaft is known in which an injection-molded portion which is formed of a synthetic resin is provided in its sliding portion. The injection-molded portion prevents the occurrence of looseness in a fitting coupling section of the steering shaft in a rotating direction.
An inner shaft and an outer shaft are coupled to each other in the axial direction by a splined structure, and the outer shaft and the inner shaft are fixed in the axial direction by shear pins which are formed of a synthetic resin.
The above-described structure is provided for dampening an axially thrusting force applied from a lower portion of the vehicle during a primary collision, e.g., a thrusting force applied from wheels against the steering shaft via a steering gear box.
A sliding member formed of a synthetic resin for supporting favorable sliding is provided between an opening portion of the outer shaft and the inner shaft in a sliding coupling section of the outer shaft and the inner shaft.
Further, the structure provided is such that shear pins formed by injection of a synthetic resin are provided at appropriate positions on the innermost inner peripheral side of the outer shaft, and the shear pins are sheared during a collision, allowing the outer shaft and the inner shaft to telescopically contract and move relative to each other.
However, the steering shafts of the conventional type have the following drawbacks. First, an appropriate clearance is provided in a splined coupling section between the outer shaft and the inner shaft so as to ensure the slidability of the shafts.
In addition, the axial slidability is made stable and favorable by means of the resin sliding member provided between the opening portion of the outer shaft and the inner shaft. Further, the resin sliding member at the opening portion of the outer shaft blocks the clearance occurring between teeth in the splined fitting section. As a result, it is possible to prevent the looseness in the rotating direction which occurs due to a slight clearance in the rotating direction, which is present between the outer shaft and the inner shaft.
However, in the operation of injecting a resin material into the slight clearance, the maintenance of precise molding conditions is required, and the margin of allowance in the setting of the molding conditions has been very strict.